1. Field of the Invention
This invention relates to the use of temperature change in determining the location and character of conducting point defects in or on flat-panel displays such as, but not limited to, those as used in laptop computers.
2. References
The following documents are incorporated by reference herein:
U.S. Pat. Nos. 5,804,980; 4,819,038; 5,334,540; 5,365,034; 5,430,305; 5,465,052; 5,615,039; 5,170,127; 5,831,392; 5,859,502.
PCT Publication Nos. WO99/36935; WO98/02899; WO99/06844; WO98/19501.
William C. O""Mara, xe2x80x9cLiquid Crystal Flat Panel Displays Manufacturing Science and Technologyxe2x80x9d (1993).
Richard M. Bozorth, xe2x80x9cFerromagnetismxe2x80x9d, IEEE Press, Copyright 1951 D. Van Nostrand Company, Inc.
Neil Ashcroft and N. David Mermin, Solid State Physics, Copyright 1976, Holt, Rinehart, and Winston.
Alexander Fetter and John Dirk Walecka, Theoretical Mechanics of Particles and Continua, McGraw Hill, Copyright 1980.
Robert M. Gray and Lee D. Davisson, Random Processes, Prentice Hall, 1986.
3. Description of Related Art
A wide variety of electronic circuits are formed as a collection of interconnections between electrical or electronic elements. On any structure which is predominantly planar, locally or globally, it is often necessary for these interconnections to overlap each other. In some cases, the overlaps can be electronic components themselves, as in capacitors, diodes, etc. During the fabrication or operation of the device, it is possible that at the overlapping points or points of near contact unintended conductive or non-conductive paths can be formed. The presence of these paths can prevent the device from operating properly. From a manufacturing or repair point of view, it is a very difficult matter to identify the location of these unintended paths. Various techniques have been employed such as direct optical examination of the surface and electrical test using contact probes and electro-optical techniques (See, for example Henley U.S. Pat. No. 5,465,052). It can be very difficult or impossible to make direct contact near a potential defect, so usually external connections are used which may make it impossible to identify the exact location of the defect. In many cases, the defect does not have an optical signature of any form on the device. On the other hand, if it were possible to identify the location of these unintended paths, many different methods exist for their repair in situ. Therefore the rapid identification of the defect locations is highly desirable.
In the case of flat-panel displays, there is typically a regular array or grid of rows and columns on the display. As used herein, a flat-panel display is a visual information display device for which the dimensions of the active display plane are much greater than the thickness in a direction perpendicular to the display plane. Examples of such devices include liquid crystal displays(LCD) and field emission displays(FED). To fabricate the array on a flat panel display, there must be many, many crossover points over a large area. Therefore, the opportunity for defects to arise is very high. It is now a common practice in the liquid-crystal display manufacturing business to find and repair defects before final assembly of the display. Unfortunately, no entirely satisfactory means of locating these defects has yet been employed.
In the case of liquid-crystal displays (LCD), there is usually a regular array of interconnections and electronics which is capable of producing an electric field. Adjacent to this array there is a molecular substance which has the property of affecting optical polarization and/or transmission in relation to the applied electric field. With an appropriate collection of optical filters, glass, and electronics, this arrangement can be used essentially as a switchable xe2x80x9clight-valvexe2x80x9d which then leads to its utility as a flat-panel display.
In the case of a field-emission display (FED), there is typically a regular array of electron-emitting devices and light emitting devices situated opposite each other. Associated with this regular array is an array of interconnections and possibly electronics which resembles that of the LCD. By applying carefully selected voltages to the rows and columns of this structure, electrons may be emitted and strike the light emitting devices thereby leading to its utility as a flat-panel display. In the case of FED displays, the invention applies to the part of the display known as the cathode from which the electrons emanate into a vacuum region.
In the case of other types of flat-panel displays, regardless of the exact nature of the light switching or generation mechanism, there is typically also a regular array of interconnections and electronics. Appropriate voltages or currents are applied to affect the optical properties of the device which then leads to utility as a flat-panel display.
In all of the above described flat panel display devices, there is a need for rapid localization of defect points during or immediately after manufacture without making repeated or microscopic contact with the device. It is desirable that this localization can be performed both during and after the fabrication of the device. It is desirable that the localization not require an optical signature of the defect and that the method be reducible to an algorithm that a computerized machine can perform without operator intervention. It is desirable that the method would allow for repair of the device on the same machine on which the localization takes place.
Because of the very large size of modern flat-panel displays, the test machines built for this purpose tend to be very large and expensive. They must operate at a very high reliability level and with an extremely high level of cleanliness. Clean room space is also very expensive in and of itself. Often, if an operator is required for the operation, especially a repair operation, the operator is stationed on a different machine entirely and the devices under test are transported from the test machine to the machine with the operator entailing great logistical and material handling costs as well as creating cleanliness problems. It may be that the required cleanliness makes a special enclosed mini-environment for the tool a necessity or practicality. Within this mini-environment, the air is typically continuously filtered and other cleanliness measures are often employed.
Most modern flat-panel manufacturing operations are actively involved in process development as well. Therefore, it would represent a significant advance if it were possible to implement a test operation on a relatively small and inexpensive apparatus even if not applicable for volume manufacturing so as to facilitate failure analysis operations on the devices under manufacture.
The present invention can be implemented with hardware much simpler and less expensive than that of most test equipment used for similar purposes. The hardware required can be made extremely reliable and may have a minimum of moving parts. The overall throughput which can be achieved is extremely high. It is possible to build simple versions of the apparatus for failure Be analysis very economically. No contact to the active area of the flat-panel display is required. The present invention also allows for localization of defective points to exceedingly high resolutions without the need for operator guidance. In light of all this, it will be appreciated that the present invention constitutes of substantial improvement over prior art.
The present invention furnishes a group of analytical techniques and systems in which localized change in temperature is employed in determining the location and nature of defects in flat-panel devices. The nature of the defects includes the electrical properties, resistance or capacitance.
In the field of semiconductor integrated circuit manufacturing, techniques exist which are often referred to as OBIC (Optical Beam Induced Current) or LIVA (Light Induced Voltage Alteration). Several apparatus have been built for semiconductor integrated circuits using laser beam, electron beam, or ion beam scanning. No application of this art has been made to the flat-panel display field, and as the physical scale, materials and processes used, and the embodiments are all different, the present invention cannot be considered an obvious extension of art in the different field of semiconductor integrated circuit fabrication.
It is well known that when dissimilar materials meet in an electrical junction, there is produced a temperature dependent electromotive force. This effect is known as the thermoelectric effect. The general name for devices based on this effect is the thermocouple. As used herein, the term xe2x80x9cthermocouplexe2x80x9d shall be understood to mean the junction formed by the close proximity or meeting of dissimilar materials without regard for whether that junction was deliberately created or the consequence of a defect or the absence of a defect in the device. The effect is material dependent, sometimes very sensitively. It is also dependent on characteristics of the material such as the alloy structure or magnetic phase of the material. When the potential is measured around an electrical circuit with dissimilar materials as described, the thermoelectric effect gives rise to a thermoelectric potential which is related to the difference in temperature at the each junction of dissimilar materials. Typically, the thermoelectric potential is on the order of 0.001 Volt for temperature differences of 100 degrees Centigrade. In general, the value of this potential is size independent down to dimensions on the scale of the electron mean free path in the material (which for metals may be on the order of 100 Angstroms or less). Considering the above description of the thermoelectric effect, it is apparent that a conducting junction is not necessarily required to observe the thermoelectric effect. For example, a capacitor formed with two different metal plates and an air gap will generate a thermoelectric electromotive force in an electrical circuit until the stored charge in the capacitor exactly counteracts the thermoelectric electromotive force.
It is also well known that the resistance of materials changes with the temperature of the material. Depending on the exact nature of the material, this change may be quite linear with temperature, or, as in the case of semiconductors, be quite rapid and exponentially dependent on the temperature. In essentially all cases, however, if a constant current flows through a material, there will be an electric field generated in the material whose magnitude will be temperature dependent. This electric field, when summed over the length of the current path, gives rise to a temperature dependent voltage drop across the material which may be measured by an external device. This principle has been used for decades for a type of optical detector known as a xe2x80x9cbolometer.xe2x80x9d In this case, an insulating junction will not flow any current and therefore the effect cannot be observed. Nothing is a perfect insulator, however, and it is to be understood that highly resistive materials such as might be found in a defect are part of the class of materials in which this effect may be used. It is also the case that capacitive currents can be significant in the presence of time varying fields and that the change in resistance may be used even if a continuous circuit is not made.
It is also well known that a material undergoing a phase transition may undergo a change in resistivity. Typically, phase transitions occur at a particular temperature and environmental condition such as pressure, applied magnetic field, or other external field in the physics sense of the word. By controlling the temperature or other environmental conditions, a phase transition may be induced in a particular material. The Curie point is an example of a ferromagnetic phase transition occurring at a temperature referred to as the Curie point or Curie temperature. For example, heating of a Nickel metal above its magnetic Curie point at about 358 degrees Centigrade causes it to undergo a phase transition to a paramagnetic state. Additions of small amounts of impurities to Nickel alloys can cause the Curie temperature to change significantly, especially toward lower temperature. As this transition occurs, the electrical resistance of the material changes abruptly.
Other examples of temperature-induced phase transitions include alloy phase transitions, or transitions between crystalline and non-crystalline forms. It should be appreciated that a phase transition can occur very rapidly and is often reversible as the temperature and environmental path to the phase transition is retraced.
Thermal energy flows through solids according to well established principles of heat diffusion in solids. In this theory, heat flows substantially according to Fick""s law of diffusion which can be stated as:             Q      .        =          k      ⁢              xe2x80x83            ⁢              (        T        )            ⁢              xe2x80x83            ⁢              ∇                  xe2x80x83                ⁢        T                                c        p            ⁢              xe2x80x83            ⁢              (        T        )            ⁢              xe2x80x83            ⁢                        ⅆ          T                          ⅆ          t                      =          k      ⁢              xe2x80x83            ⁢              (        T        )            ⁢              xe2x80x83            ⁢              Δ        2            ⁢              xe2x80x83            ⁢      T      
where {dot over (Q)} is the vector flow of heat
k(T) is the thermal conductivity of the material in question at its temperature
cp(T) is the heat capacity of the material in question at its temperature
T is the temperature at a given point.
Small modifications to this law do occur, but under most circumstances these effects can be neglected or they can be incorporated into the largely phenomenological constants cp(T) and k(T). Fick""s law permits the calculation of the characteristic durations which are required for heat to diffuse from a point, area, or volume source in or on a solid, such as a flat-panel display. Typically, for glasses used for flat-panel display manufacturing, the thermal conductivity k(T) is approximately 1 W/(m deg C) and the heat capacity is approximately 2xc3x97106 J/(m3xc2x0 C.). In the event that flat-panel displays are manufactured on other substrates, such as quartz or ceramics, substantially different values may be obtained. It is a straightforward matter to show that the radius which heat will substantially diffuse in a given time in glass of this sort is on the order of
r=0.0014 sqrt(t) meters,
where t is in seconds. Therefore, the characteristic time for 10 micron diffusion is about 50 microseconds. Similarly, for 100 micron diffusion, the time is about 100 times longer or about 5 milliseconds. These times are very convenient for testing flat-panel displays because they provide for many thousands of independent testing events per second at the size scales present on flat-panel displays without undue thermal diffusion which would increase the required thermal energy input as well as reduce the spatial resolution of the invention. For any given situation, a calculation of the thermal diffusion characteristics in both space and time has to be done to determine an efficient temporal and spatial profile of thermal energy excitation of the device under test.
In accordance with one aspect of the invention, a change in the temperature within a first region of a display panel or grid results, if certain electrical conduction paths exist, in a voltage and/or current change in a signal which may be electrically measured locally by direct contact or by contact to external electrical connections on the device. In some embodiments the voltage and/or current change (referred to collectively herein as a power change) occurs only if the subject conduction paths exist within the region of the temperature change, and in other embodiments the subject conduction path does not need to be within the region of temperature change in order for the power change to appear. Also, in some embodiments the presence of the subject conduction paths indicates a defect (e.g. a short), wherein in other embodiments the absence of a current path indicates a defect (e.g. an open-circuit type defect).
The power change measured will be characteristic of the temperature change, the temperature of the device, the electrical properties of the device, the electrical measuring apparatus, and the duration and temporal history of the temperature change. Because of the potentially complicated electrical properties of the device and the diffusion of heat within the device, a processor is normally attached to the electrical measuring apparatus to make the determination of the existence or non-existence of a subject current path and its location. It will be understood that the location and magnitude of the temperature change may change with respect to the device in time and in position with respect to the wafer. It will also be understood that the application of thermal energy is meant to be thermal energy delivered to or removed from the device under test and not the form of the energy delivered from the external apparatus. In many cases, the form of the energy delivered will not be thermal, as in the case of a laser with electromagnetic light energy. Nevertheless, the energy will be deposited in the device in the form of thermal energy.
In accordance with a further aspect of the invention, thermal energy is transferred to or from the device by external apparatus to effect the necessary temperature change. In one embodiment, a laser is focused on the surface of the device for the purpose of transferring thermal energy. It is to be understood that there are many methods of transferring thermal energy such as inductive coupling, infra-red radiation, and others, all of which are referred to herein as ways of inducing a temperature change within the device under test.
In accordance with a further aspect of the invention, the device under test itself may be designed so as to change the electrically observed effect. In particular, the device may be designed to provide increased power effects or reduced noise in the observed electrical signal, thereby increasing system sensitivity. These design changes may include, but are not limited to, changes in the geometry of the interconnections and electronic components of the device, changes in the composition or preparation of some or all or parts of the interconnections and electronic components of the device.
In accordance with a further aspect of the invention, the application of thermal energy may be by an optical beam or a laser. In this case, the beam may be moved across the device, or the device may be moved with respect to the beam or both.
In accordance with a further aspect of the invention, the application of thermal energy may be by the application of hot or cold gas or fluid, such as air, nitrogen, helium, argon, freon, alcohol, etc. to a localized region of the device. For example, an air knife is a frequently used device for cleaning the surface of flat panels. Such a device if combined with a heater would be capable of raising the temperature of the device in a local region which would permit the localization of defects. It is possible to construct a similar hot gas source which would be effective over a very restricted area, thereby imparting high spatial resolution to the defect location.
In accordance with a further aspect of the invention, when the application of thermal energy is by an optical beam or a laser, the intensity on target may be increased with a multi-pass or resonant structure thereby allowing for increased absorption of laser energy. It is typically the case that metal films have high reflectances, of the order of 85 percent. As a result, most of the optical energy applied to the device under test will be reflected and not useful for heating the device. By re-reflecting the reflected beam so as to impinge on the target twice or more, it is possible to greatly increase the amount of optical energy absorbed, or conversely, to greatly reduce the amount of optical energy required. It will be appreciated that this can result in a significant practical benefit.
In accordance with a further aspect of the invention, the application of thermal energy at one position on the device may result in a change of the measured current or voltage indicative of a defect at the same or a different position on the device. For example, if an anomalous current flows in the device due to a defect, the position or magnitude of the anomalous current could signify the location of a defect different from where the current flows or does not flow, as the case may be.
In accordance with a further aspect of the invention, the absence of a voltage or a current could be used for the purpose of detecting the presence of or location of defects. For example, it is commonly the case that open circuit defects are produced in the manufacture of LCD displays. In this case, the absence of a voltage when a voltage would be expected for a functional device could indicate the presence of an open-circuit-type defect.
In accordance with a further aspect of the invention, the localization of the defects may take place by doing an exhaustive search of the entire device or substantially all of the device. An exhaustive search is one in which thermal energy is applied to every point of the device in sequence and therefore every point on the device is tested for the presence or non-presence of a defect.
In accordance with a further aspect of the invention, the localization of the defects may be accomplished by using a binary search technique. By inducing a temperature in successively reduced areas of the device, it is possible to localize the defect to an area whose size decreases exponentially with the time spent localizing. For example, one half of the device could be tested in accordance with the invention. At this point the presence of a defect in this half would necessitate the test of one half of the already tested half By repeating this process within the tested half and then repeating on the other half, always terminating when the absence of a defect is found within the tested area, a search with exponential resolution may be realized. It will be appreciated that practical factors may preclude a strictly binary search, but other search algorithms which may include a progressive narrowing component, are intended to be within the spirit of the invention.
In accordance with a further aspect of the invention, the localization of the defects may take place by using a coarse search followed by a fine search or any combination of the above methods with a coarse and fine search. A coarse and fine search is often the preferred embodiment for large panels. In this case, a search of all rows and all columns is made to determine those rows and columns likely to contain defects. A list is formed of likely rows and columns based on the probability of occurrence of a defect at a particular row and column and the areas so described are inspected in an order chosen by a suitable algorithm. The points are inspected by the above described techniques and search methods, or by conventional methods, until either a repair action can be attempted, no defect is found, or the device is determined to be unrepairable.
In accordance with a further aspect of the invention, the localization of the defect may occur within a very small region of the device under test. For example, another method may be utilized for determining a rough location of a defect and the present invention can be used to localize the defect to a smaller area.
In accordance with a further aspect of the invention, the energy transferred to the device can be controlled by measuring the energy emanating from the device. For example, if a laser were used to apply the thermal energy, the energy in the reflected laser light could be measured for the purpose of controlling the temperature change at the device. Similarly, with appropriate filters, infra-red radiation from the surface could be used to measure the temperature change at the device even in the presence of a strong laser beam.
In accordance with a further aspect of the invention, the points to be probed for defects need not be visible from either the front or back surface of the device. Similarly, the thermal energy may be applied from any direction. In particular, in the case of a laser used to apply the thermal energy, the laser may be applied from the back surface of the device. It is not necessary that the actual defect point be visible as thermal energy will diffuse into the device in a predictable and repeatable fashion.
In accordance with a further aspect of the invention, it is possible to measure either the voltage or the current or a combination of both depending on the electrical network to which the device is attached. The practical implementation will often determine which or what combination of these measurements is most efficacious. In the case of pure current measurement, it is desirable to apply a fixed or substantially fixed voltage bias to the device under test while the current measurement is performed. In the case of pure voltage measurement, it is desirable to apply a fixed or substantially fixed current bias to the device under test while the voltage measurement is performed. In the case of a combination measurement, the choice of combination of applied voltage and current bias can be made according to the well established principles of electrical circuit theory. It will be appreciated that for certain embodiments, such as those relying on a thermoelectric effect, no bias at all may be required.
In accordance with a further aspect of the invention, the applied voltages, currents or thermal energy to the device under test may be varied in time. Variation of the thermal energy application in time can take the form of varying the magnitude of the energy application, the position of the energy application with respect to the device under test, or a combination of both variations. The variation of the applied voltages, currents, or thermal energy to the device under test can in many situations be utilized to increase output signal, reduce noise, or increase the rate at which the test and localization can be performed.
In accordance with a further aspect of the invention, test apparatus for implementing the present invention can be designed so that it attaches to or is aligned with another already present apparatus such as a microscope or other flat-panel inspection or repair device (inspection device). The test apparatus can be designed so that an internal thermal energy source, such as a laser, can be directed to the device under test by the inspection device. Provision for electrical connections to the device under test are made. Internal to the test apparatus the thermal energy source may be directed to selected locations within the field of view of the inspection device, or the inspection device can be directed to move the device so as to move the location of the thermal energy source relative to the device, or an operator can manually achieve this relative motion either independently or at the direction of a processor within the apparatus. In this manner, the defect points on a flat-panel display may be rapidly localized with an easy to construct and very low cost addition to a pre-existing inspection device. This aspect of the invention may be used in conjunction with any combination of the above described aspects of the invention.
The present techniques for localizing and measuring characteristics of defects are quite simple. The components of the invention are commercially available and compatible with the conditions under which the devices described above are manufactured and assembled. The test process can be implemented in an entirely automated approach and can be extremely rapid. The technique does not require physical contact except to external connections on the device where contact presumably would be made eventually anyway. There are many ways that the thermal energy may be applied to the device. Consequently, the invention provides a large advance over the prior art.